He expertly sprayed a trio of bursts at the alien.The Elite roared in defiance and fired in return, but the sheer volume of the specially hardened 7.62mm projectiles caused the the Elite's shielding to flare, overload and fail. The bulky alien fell to his knees, bent forward, and collapsed.P.32

No idea how many bullets were in each burst but I would have assumed 3-5.--------------------------------------------------

A single burst kills a Jackal:

He fired a burst at the Jackal's unprotected flank and the alien hit the deck-plates, dead.P.33

Again, the word "Burst" is used making it difficult to know exactly how many bullets he fired.--------------------------------------------------

Takes down a Grunt with a melee and a three round burst:

The Master Chief took three steps forward, used the assault rifle to knock the creature off it's feet , and followed it with a three-round burst."P.40

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One magazine can damage a Banshee:

Careful not to fire early, the Spartan waited for the Banshee to come within range, then squeezed the trigger. The first assault ship came straight at him.which made it relatively easy to stay on target. Bullets impacts sparked on the Banshee's hull as his ammo counter dwindled. The ship shuddered as at least some of the armor piercing rounds penetrated the fuselage, pulled up out of its dive, and started to trail smoke.P.58

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After putting in a second magazine, he destroys the Banshee:

The Covenant ship ran into the stream of bullets, exploded into flames and slammed into the cliff wall."P.59